1. Field of the Invention
The invention relates to a method of forming a magnetic layer by reactive sputtering, and more specifically to a method of forming a magnetic layer of a perpendicular magnetic recording medium for use in a hard disk drive or the like, a magnetic recording medium, and a magnetic recording and reproducing apparatus.
Priority is claimed on Japanese Patent Application No. 2008-291031, filed Nov. 13, 2008, the content of which is incorporated herein by reference.
2. Background Art
In recent years, magnetic recorders such as magnetic disk units, flexible disk units, and magnetic tape units have been used over a remarkably wider range of applications, and play more important roles. With this trend, an attempt is being made to highly increase the recording density of magnetic recording media for use in such recorders. In particular, the surface recording density of a HDD (hard disk drive) has been vigorously increasing since the introduction of an MR head and PRML technology. Moreover, since a GMR head and a TuMR head have also been introduced in recent years, the surface recording density keeps increasing at a rate as high as about 100% per year.
Meanwhile, as for a magnetic recording system of a HDD, a so-called perpendicular magnetic recording system has become popular rapidly in recent years as a substitute technology for a conventional longitudinal magnetic recording system (a recording system in which the magnetization direction is oriented parallel to the substrate plane).
In the perpendicular magnetic recording system, crystal grains in a recording layer in charge of data-recording have their easy-magnetization axes perpendicular to a substrate. The term “easy-magnetization axis” refers to an axis along which magnetization occurs easily. In the case of a typical cobalt (Co) alloy, an axis parallel to the normal line of the (0001)-plane in the Co hcp structure (c-axis) serves as the easy-magnetization axis.
In the perpendicular magnetic recording system, the easy-magnetization axes of magnetic crystal grains are vertical. Therefore, this system features a resistance, even if higher recording densification is advanced, against influences from demagnetizing fields which are formed between recording bits, and thus is magnetostatically stable.
The perpendicular magnetic recording medium typically comprises a structure in which a ground layer, an inner layer (orientation control layer), a magnetic recording layer, and a protective layer are formed in this order on a nonmagnetic substrate. In addition, on completion of the formation of the protective layer, the surface is often coated with a lubricant layer.
Moreover, in many cases, a magnetic film called a soft magnetic underlayer is provided under the ground layer. The ground layer and the inner layer are formed in order to improve the properties of the magnetic recording layer. Specifically, they act to align the crystal orientation of the magnetic recording layer as well as regulating the shape of the magnetic crystal.
For the higher recording densification of the perpendicular magnetic recording medium, it is necessary to concurrently achieve thermal stability and a low-noise state. Two methods are usually employed to reduce noise. The first one is a method in which magnetic crystal grains in the recording layer are magnetically separated and isolated to thereby reduce the magnetic interaction between magnetic crystal grains. The second one is a method in which the sizes of the magnetic crystal grains are made smaller. Specifically, for example, a method can be enumerated in which SiO2 or the like is added to a recording layer to thereby foam a perpendicular magnetic recording layer having a so-called granular structure, where magnetic crystal grains are surrounded by SiO2-rich grain boundary regions or the like (for example, see Japanese Unexamined Patent Application, First Publication No. 2002-342908).
As a method of forming a perpendicular magnetic recording layer having such a granular structure, a method has been disclosed in which a recording layer of a granular structure is formed by using a composite target which contains a CoCrPt alloy and SiO2 by DC magnetron sputtering in an argon-oxygen mixture gas atmosphere (IEEE Transactions on Magnetics, Vol. 40, No. 4, July 2004, pp. 2498-2500). This document reported that the coercive force was enhanced and the recording-and-reproducing properties were improved by performing reactive sputtering in an oxygen-containing atmosphere.
In addition, it is also reported that the optimum oxygen partial pressure is determined by the SiO2 concentration, that the optimum oxygen partial pressure increases as the SiO2 concentration decreases, and that the magnetic properties and the recording-and-reproducing properties are considerably impaired when the oxygen concentration becomes excessive beyond the optimum value.
Furthermore, Japanese Unexamined Patent Application, First Publication No. 2004-346406 disclosed a sputtering apparatus capable of uniformizing the film thickness, the film quality, and the film characteristics by, upon introduction of a sputtering gas and a reactive gas into a vacuum chamber in order to form a film by reactive sputtering, uniformizing the concentration of the reactive gas which flows along the surface of a target and thereby uniformizing the reaction between the reactive gas and the target. This apparatus is a sputtering apparatus for depositing a film on a substrate by arranging a cathode having at least one target so that the cathode faces the substrate, and sputtering the target on the basis of reactive sputtering, wherein the apparatus comprises a central gas-introduction mechanism by which the reactive gas supplied from a reactive gas feeding device flows outward from the central part of the cathode unit along the surface of the target.
Incidentally, upon such reactive sputtering in an oxygen-containing atmosphere, an event can be seen in which the concentration of oxygen to be taken into the magnetic layer varies depending on the position on the disk, because the distribution of the oxygen radical concentration is nonuniform in the sputtering chamber. This causes an inconvenience in that it is difficult to achieve uniformization of the magnetic properties and the recording-and-reproducing properties all over the entire surface of the disk.
As a method for solving this inconvenience, the 2nd patent document has disclosed a method in which a reactive gas is allowed to flow outward from the central part of a cathode unit along the surface of the target.
However, although this method can provide the reactive gas per se uniformly to the substrate surface, because of the gas flow, a plasma space formed in the vicinity of the substrate surface flows to the downstream, which makes the plasma space unstable.
Moreover, in this method, the gas flows in between the substrate and the plasma, where a plasma-free space is formed. This plasma-free space inconveniently cancels oxygen radicals being ejected from the plasma space to the substrate surface. This results in a deceleration of the film formation by reactive sputtering, as well as causing a nonuniformity of the magnetic film deposited on the substrate surface and a nonuniformity of the film thickness distribution.
Under such circumstances, there has been a demand for a magnetic layer formation method capable of uniformizing the distribution of the oxygen radical concentration upon reactive sputtering in an oxygen-containing atmosphere. However, in the current situation, an effective and appropriate method has not yet been provided.